Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly, to an implantable rate-responsive pacemaker that generates stimulation pulses on demand at a rate determined by a sensor indicated rate (SIR) signal, where the SIR signal has a selectable response to various types of patient activity.
A pacemaker is a medical device that generates and delivers stimulation pulses to a patient's heart in order to cause the heart to beat in accordance with a prescribed sequence and pattern. Usually, the prescribed sequence and pattern is simply a prescribed pacing rate, with the pacemaker providing stimulation pulses only when the patient's heart fails to naturally beat on its own at a rate that is at least as fast as the prescribed pacing rate. Thus, if the pacemaker fails to naturally beat within a prescribed time interval (usually referred to as the "escape interval"), which prescribed time interval represents the period of the pacing rate, then and only then does the pacemaker step in with a pacemaker-generated stimulation pulse in order to force a heartbeat. In this manner, pacemaker-generated stimulation pulses are provided by the pacemaker only when needed, i.e., "on demand," when the heart is unable to beat on its own.
Most modern pacemakers are programmable, meaning that various parameters associated with the operation of the pacemaker may be noninvasively selected ("programmed") after the pacemaker has been implanted. Such programming is achieved through the use of an appropriate external programming device and telemetry circuit that establishes a suitable communication link with the memory circuits of the pacemaker. Examples of programmable, implantable pacemakers may be found, e.g., in U.S. Pat. Nos. 4,223,679 and 4,686,988; and in Furman, et al., A Practice of Cardiac Pacing, (Futura Publishing Co., Mount Kisco, N.Y.--1986).
In recent years, many pacemakers have also included rate-responsive pacing capabilities, meaning that the pacing rate is automatically adjusted as a function of some sensed physiologic-related parameter. Thus, such pacemakers include one or more sensors adapted to sense an appropriate physiologic-related parameter, such as physical activity, respiration rate, blood oxygen saturation, blood pH, blood temperature, or the like (any or all of which may provide some indication was to what the patient's heart rate should be), and appropriate signal processing circuitry for converting the sensed physiologic-related parameter to an appropriate control signal that controls the pacing rate. A pacemaker having such rate-responsive capability is referred to as a "rate-responsive pacemaker," and the control signal that controls the pacing rate as a function of the sensed physiologic-related parameter(s) is usually referred to as the "sensor indicated rate" (SIR) signal.
There are many types of rate-responsive pacemakers known in the art. Some utilize a single sensor in deriving the SIR signal, as described, e.g., in U.S. Pat. Nos. 4,140,132; 4,202,339; 4,428,378; 4,576,183; and 4,940,052. Others use multiple sensors, as taught, e.g., in U.S. Pat. Nos. 4,722,342; 4,782,836; and 5,097,831. Whether one or multiple sensors are used, there is a need to process the signal(s) generated by the sensor(s), which signal(s) is typically referred to as the "raw sensor signal," in an appropriate manner so that an SIR signal results that is truly indicative of a beneficial pacing rate.
The manner in which the raw sensor signal is processed can have a great effect upon the sensitivity of the SIR signal. Rate-responsive pacemakers that utilize the type of activity sensor and signal processing described in U.S. Pat. No. 4,428,378, for example, tend to produce an SIR signal that is overwhelmingly responsive to pedal impacts (walking signals) and relatively unresponsive to arm movements. This is apparently because, as described in the '378 patent, the raw sensor signal is filtered and peak amplitudes above a certain threshold level are averaged. Thus, as each step is taken, the sudden jolt or impact caused by the foot striking the ground tends to be more dominant in the processed sensor signal than are the somewhat less sudden and smoother portions of the raw sensor signal caused by arm movement. Disadvantageously, an SIR signal that is dominated by the pedal impact portions of the raw sensor signal may not always provide an indication of the most beneficial pacing rate. Thus, what is needed is a rate-responsive pacemaker wherein the dominance of sensed pedal impacts in determining the SIR signal may be selectively controlled.
In contrast, rate-responsive pacemakers that utilize the type of activity sensor and signal processing described in U.S. Pat. Nos. 4,940,053 or 5,040,535, tend to produce an SIR signal that is overwhelmingly responsive to arm movement of the arm on the side of the patient where the pacemaker is implanted. The raw sensor signal is processed in these patents to produce an SIR signal that is based on the energy content of the raw sensor signal or the average area of the raw sensor signal. Unfortunately, an SIR signal that is heavily dependent on arm motion, particularly arm motion of only one arm, may also not represent the most beneficial pacing rate. Hence, what is needed is a rate-responsive pacemaker wherein the SIR signal is not dominated by arm motion, particularly arm motion of just one arm.
In view of the above, it is thus evident that what is needed is a rate-responsive pacemaker wherein the raw sensor signal can be suitably processed to emphasize a selected aspect thereof, with the results of such processing thereafter being appropriately weighted in the formation of the SIR signal, thereby selectively controlling the sensitivity of the SIR signal to various sensed events, e.g., pedal impacts and/or arm motion.